Massive Unknown Precursors in Wastewater Treatment Plants: A Significant Source of Ultrashort-Chain Perfluoroalkyl Substances in the Receiving River

-perfluorous nonenoxybenzenesulfonate is the more frequently detected emerging PFAS. PFAS concentrations increase in most WWTPs after treatment but decrease in some WWTPs, especially M5 adding membrane bioreactor to the conventional treatment process. The 5.15-1265% increase in PFAA molar concentrations after the TOP assay reveals substantial precursors in wastewater. TFA accounts for over 90% of the total increase in PFAAs in some WWTPs. However, more than 99% of TFA increments cannot be explained from the known precursors. The annual discharge of TFA contributes 50% for the total PFAS emission flux (8993 kg/a), which should be a concern.